Stem rust (Puccinia graminis f.sp. avenae)is a fungal foliar disease of oats that can cause up to 90% yield loss and also reduces grain quality in susceptible varieties. Widespread outbreaks are very damaging but rare. Regional outbreaks are more common, causing losses over limited areas. Stem rust is adapted to warmer conditions than leaf rust and is usually detected later in the season (mid-spring). Disease severity can increase extremely rapidly once a crop is uniformly infected.